Me and many others know most about the mythology as it is depicted in the Kalevala. However it’s worth bearing in mind that Kalevala is a fictional book written by Elias Lönnrot, based on the oral tradition he collected. He has linked a more or less chaotic bunch of tales, poems and songs into a more or less coherent story. For example he has combined several characters from the legends into some of his characters.
Furthermore the oral tradition he and others collected was at the time from people who had lived with Christianity and as Christians for generations. So, a lot of what we have now is pretty close to being made up. In addition to the oral collectors’ archives, there are some descriptions from an earlier time where priests describe the horrible pagan beliefs and wrong gods the Finns respected at the time. Those are obviously very biased
but they describe the situation from a closer moment in time.
There is a religious community of the ancient Finnish faith, the Karhun Kansa, and they consider Kalevala to be interesting fiction. I have tried to find out about their actual beliefs and it seems they are more or less neopagan, they don’t have strict rules, texts or rituals but rather have the view that each person will find the connection to the spirits/gods/powers in a way that suits that person. Alternatively, they just say this in interviews and actually do have elaborate rituals! But their celebrations are open to others too, so I find it unlikely that they would keep secret bear mass in hiding
I may be wrong, I don’t personally know any of them (and they are very few, only a couple dozen actual members).